As a tunable laser capable of changing wavelengths in a wide wavelength range, there is a laser obtained by combining a semiconductor optical amplifier (SOA) and a vernier-type filter capable of selecting any wavelength using a combination of two filters each having a periodical wavelength selection characteristic. The tunable laser using the vernier-type filter includes, for example, a laser using two ring resonators and a laser using two sampled grating distributed Bragg reflectors (SG-DBRs) as filters.
In a tunable laser using an SOA and a wavelength filter including two ring resonators each of which is formed by a waveguide including a silicon waveguide core, when current applied to the SOA is increased and a laser output is increased, the operation becomes unstable. This is caused by the following reasons. When the laser output is increased, high intensity light is propagated to the silicon waveguide including the silicon waveguide core inside the laser resonator, and two-photon absorption occurs, so that photo careers are collected in the silicon waveguide, thereby changing an equivalent refractive index of the silicon waveguide. In addition, the change in the equivalent refractive index of the silicon waveguide causes fluctuation of the resonance wavelength of the ring resonator, which is determined based on the product of the equivalent refractive index and a circumference of the ring resonator. As a result, the oscillation wavelength of the laser is fluctuated, and the oscillation spectrum is widened, and multi-mode oscillation occurs, so that the operation becomes unstable.
Therefore, in the tunable laser using the SOA and the wavelength filter including two ring resonators each of which is formed by the waveguide including the silicon waveguide core, it is difficult to achieve the high output.
The followings are reference documents.    [Document 1] International Publication Pamphlet No. WO2003/023916 and    [Document 2] International Publication Pamphlet No. WO2007/029647.